Inkjet printing is a type of printing that propels drops of ink (also referred to as droplets) onto a medium, such as paper, a substrate for 3D printing, etc. The core of an inkjet printer includes one or more print heads (referred to herein as inkjet heads) having multiple ink channels arranged in parallel to discharge droplets of ink. A typical ink channel has elements including a nozzle, a chamber, a narrow channel for feeding ink into the chamber (restrictor), and a mechanism for ejecting the ink from the chamber and through the nozzle, which is typically a piezoelectric actuator connected to a thin, flexible diaphragm which forms part of the chamber wall. The parameters of the channel elements, size, geometry, material properties, etc., together with the fluidic properties of the ink all play a role in determining the properties of the jet, drop size, drop velocity, ligament structure, maximum frequency, etc.
To discharge a droplet from an ink channel, a drive circuit provides a jetting pulse to the piezoelectric actuator of that ink channel. In response to the jetting pulse, the piezoelectric actuator pushes on the diaphragm generating a momentary high pressure inside of the ink channel to push the droplet out of the nozzle. The jetting pulse has a drive waveform designed in conjunction with the inkjet head channel elements and ink parameters to control how droplets are ejected from each of the ink channels. The drive waveform of the jetting pulse is thus designed to optimize performance for each head, ink, and application.
One consideration in the design is that, in addition to the desired momentary high pressure inside the chamber, the drive waveform also excites two chamber resonances known as the Helmholtz and Slosh modes resulting in undesirable pressure oscillations and a long recovery time inside the chamber following the expulsion of the droplet. This “ringing” and slow exponential recovery of the ink meniscus can persist in a channel for a long enough time that chamber equilibrium will not have been reached by the time of the next firing required for that channel. The next firing can thus generate a droplet having a different volume/velocity and stability from that of the preceding drop.
In the past, this problem has been addressed in two ways:
(a) The damping of the ringing can be increased by making the total resistance in the channel somewhat larger. This can be done by increasing the resistance of the restrictor and the orifice. It should be noted that the Helmholtz damping is controlled by a resistance, RH, which is the parallel combination of the restrictor Rr and the orifice Ro:RH=RrRo/(Rr+Ro).When the orifice resistance is made very large: RH→Rr as Ro→∞. When the restrictor resistance is made very large: RH→Ro as Rr→∞. However, the Slosh mode damping is controlled by a resistance, Rs, which is the series combination of Rr and Ro:RS=Rr+Ro In most cases the Slosh mode frequency, S, is much lower than H and also RS is close to critical damping. For RS>=critical damping, increasing RS will only serve to increase the time for meniscus recovery. In practice we see that after firing, the meniscus returns exponentially and slowly under the Slosh mode with a damped Helmholtz oscillation riding on the return. The best results for minimum variation of drop velocity/volume with frequency are obtained from a compromise between lower Slosh damping and higher Helmholtz damping.
(b) The drive waveform can be designed with a segment of the waveform in which the meniscus Helmholtz ringing is driven 180° out of phase with its motion (clamping). However, because the equations describing meniscus recovery are non-linear, the timing of an out-of-phase segment is also important. For example, when the meniscus first starts to return to its rest position from a deep retraction, the recovery is initially governed mostly by the Helmholtz oscillation and is relatively rapid. This allows the possibility of allowing an initially uninterrupted rapid recovery before starting the out-of-phase segment having a “braking pulse” to avoid overshooting just before full recovery is reached.
Printing speed is directly dependent upon the number of jets and the maximum jetting frequency of the jets. Therefore, a high maximum jetting frequency is beneficial in that higher printing speeds are provided to customers. However, jetting at high frequencies requires a short time interval between jet firings resulting in drop velocity and drop mass which exhibit the largest fluctuations with frequency. The amplitude of these large fluctuations at high frequencies leads to errors in the volume, shape, and position of the drops deposited on a print medium. Presently, to determine the maximum jetting frequency of an inkjet head, the inkjet head is tested by firing a jet on a test stand at a constant frequency, and measuring drop velocity and/or mass. The frequency is slowly increased until the jet fails. The frequency at which the inkjet head fails is considered the maximum jetting frequency of the inkjet head. Tests such as this are commonly used to define a limitation on the maximum jetting frequency, which in turn, may limit the printing speed of the inkjet head. It can therefore be concluded that the old method of determining the maximum operating frequency is unnecessarily restrictive.